Casting method for aluminum or aluminum alloys and a mold therefor

ABSTRACT

A method of casting aluminum or aluminum alloys characterized by utilizing a mold of ferrous or ferrous alloy members having at least a portion of the mold surface provided with a carbide layer of at least one metal selected from the Group V, Subgroup a of the periodic table. In particular, the metal carbide layer can be a vanadium carbide, a niobium carbide, or a tantalum carbide, which layer is preferably a diffusion layer on the ferrous alloy mold member having a thickness in the range of 2 to 100 microns.

[ 1 Nov. 19, 1974 3,680,626 8/1972 Kusunoki et 164/72 X FOREIGN PATENTS OR APPLICATIONS 489,266 7 1938 Great Britain....................'.... 164/72 836,421 6/1960 'Great Britain........................ 164/72 Primary Examiner--Andrew Juhasz- Assistant Examiner-John E. Roethel Attorney, Agent, or Firm-Hil1, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [57] ABSTRACT A method of casting aluminum or aluminum alloys characterized by utilizing a mold of ferrous or ferrous alloy members having at least a portion of the mold surface provided with a carbide layer of at least one metal selected from the Group V, Subgroup a of the periodic table. In particular, the metal carbide layer' can be a vanadium carbide, a niobium carbide, or a tantalum carbide, which layer is preferably a diffusion layer on the ferrous alloy mold member having a thickness in the range of 2 to 100 microns.-

I 9 Claims, 2 Drawing Figures CASTING METHOD FOR ALUMINUM OR ALUMINUM ALLOYS AND A MOLD THEREFOR [75] Inventors: Noboru Komatsu; Tohru Arai;

Masayoshi Mizutani, all of Nagoya,- Japan 7 [73] Assignee: Kabushiki Kaisha Toyota Chuo Kenkyusho July 2, 1973 Appl. No.: 375,390

Foreign Application Priority Data I July 11, 1972 47-69767 [52] U .S. Cl.'.....................249/115, 117/53, 164/72 [51] Int. B22c 3/00 [58] Field of Search 164/57, 58, 72, 138; Y 249/115; 117/53 [56] References Cited I 9 UNITED STATES PATENTS 3,666,531 5/1972 Cocks 1 17/53 United-States Patent Komatsu et :11. I v

[22] Filed:

i'E'l'El' l 1 The present invention is directed to a method of casting aluminum alloys, a mold used in casting, and a method of preparing the mold.

In either die castingor gravity casting of aluminum or aluminum alloys, casting molds made of metal such as alloy steel are usually employed. However, since the molten aluminum or aluminum alloy has a strong tendency to corrode and dissolve metals used for molds, corrosion and erosion of the mold surfaces easily occur during casting operation and adhesion of the molten aluminum to the mold surface will also occur. As a result of corrosion of the mold surface and adhesion of molten aluminum to the mold surface, various problems arise. Forexample, changes in the surface condition of the mold decrease the efficiency of removal of the casting from the mold, and produce rough surfaces on the casting. In additionto these problems, the cast ing produced in the mold will have poor quality such as poor dimensional accuracy and an increased amount of time is required to repair the worn molds or mold surfaces.

In order to solve these problems, selectionof mold. materials and surface treatment such as plating or nitriting of the mold surfaces have been suggested. Although some improvements due to these'suggestions have occurred, manyunsolved problems still exist in the method of casting aluminum.

The present invention is directed to a method of castingaluminum oraluminum alloy to obtain castings with high dimensional accuracy, smooth casting surfaces and high quality by utilizing a mold having mold surfaces which obviate adhesion of the aluminum to the mold surface and'reduce wear of the ,mold surfaces. The mold .is formed of mold members of a ferrous metal as the base material and have a portion of the surface which is in contact with the molten aluminum or aluminum alloy formed with a metal carbide layer of at least one metal selected from Group V, Subgroup a of the-periodic table. The metal carbide layer is preferably formed by a diffusion process comprising heating the mold member made of a ferrous metal containing carbon and subjecting the surface to be provided with the metal carbide layer to a powdery treating material,

a treating fused bath or a gaseous treating atmosphere.

ON THE DRAWINGS FIG. 1 is a cross-sectional view of a mold utilized for aluminum die casting in accordance with the present invention; and p FIG. 2 is a photomicrograph showing a niobium carbide layer formed on vthe surface of a core pin of a mold in accordance with the present invention The principles of the present invention'are particularly useful in die casting aluminum or aluminum alloy in a mold illustrated in FIG. 1 which mold includes a plurality of mold members for example a fixed die] and a movable die 2. The movable die 2 has ejection pins 3 and a core pin 4. The dies 1 and 2 each have surfaces which coact to form a gate 5 to a mold cavity formed by the two die members 1 and 2. To introduce the molten aluminum or aluminum alloy into the mold, the fixed die 1 is provided with a sprue 6.

Prior art molds having a configuration of the mold illustrated in FIG. 1 were formed of steel alloys such as a quenched and tempered tool steel. The areas around and near the gate 5 were subjected to wear or erosion due to the fast flow speed of the molten aluminum during casting operation. Also projecting parts-such as a core pin head 41 of core pin 4 and the projection 11 in the fixed die I have a poor heat conduction and were subjected to both erosion and adhesion of the molten aluminum during casting. In addition to the wear of the above-mentioned portion of the cavity, the heads 31 of the ejection pins 3 were subjected to wear when pushing or removing the casting from the separated die members I and 2.

The present invention provides a metal carbide layer at least on those portions of the mold surface which are subjected to erosion, corrosion or wear -If desired, the entire mold surface can be provided with the metal carbide layer. The carbide layer is a metal carbide of at least one metal selected from Group V, Subgroup a of the periodic table, and includes such metals as vanadium, niobium and/or tantalum. These metals form carbide layers of vanadium carbide (VC), niobium carbide (NbC) and tantalum carbide (TaC) respectively. Each of these metal carbides has great hardness, (hv: 3,000) and. high wear resistance during aluminum casting. Therefore, a mold for casting aluminum or aluminum alloy having a surface area provided with the abovementioned metal carbide layer has excellent durability. Thus, it is possible, to obtain a high quality casting of aluminum or aluminum alloy both efficiently and economically by using the mold of the present invention.

Due to manufacturing costs, ease of handling and strength of the mold members, the mold members such as thedies l and 2 are formed of a ferrous metal containing carbon such as carbon steel or tool steel. In addition to the existing properties of the ferrous metal mold members, the metal carbide layer of the present invention adds the properties of high corrosion resistance and high wear resistance.

The metal carbide layer may be formed on either a portion of the mold surface or the entire mold surface by various methods. An example is a diffusion method 'which includes'keeping a treating material containing the metal in contact withthe mold surface at a high temperature to form a diffusion layer by diffusion of the metal into the mold surface. The carbide layers could also be formed by spraying. However, with the spraying method of providing the carbide layer, the metal carbide layer is only bonded to the base metal of the mold by a mechanical bond. Thus, the mold provided with a carbide layer by a method of spraying will not be suited for use as a casting mold since the carbide layer will easily peel off especially from thermal shock which occurs due to repeated heating and cooling during use of the mold. With the diffusion method of applying the metal carbide layer, a metallic bonding is obtained between the layer and the substrate of ferrous metal. Due to the metallic bonding, the layer-does not easily peel off the substrate even when subjected to repeated stresses due to the difierent coefficients of thermal expansion for the base metal of the mold and the metal carbide layer.

The diffusion method for preparing the mold surfaces may include any method such as a pack method which uses a powdery treating material, a gaseous method which treats the surface in a gaseous atmosphere, a fused salt method which treats the surfacein a high temperature fused salt or an electrolytic method which electrolytically treats the surface while in a fused salt.

' fused into the surfaces of the mold and form the carbide layer on the surfaces of the mold.

In one of the methods using a powdery treating material, either the entire surface or portions of the surface may be treated. In this method, the surface 'to be treated is contacted with a powdery treating material consisting of a powder of the metal and a fluoroborate such as potassium tetrafluoroborate either by being packed in the material or having the treatment material spread over the specific part of the mold surface to be treated. Then the entire arrangement is heated with the fluoroborate which activates the metal and the metal in the material diffuses into the surface and acts with the carbon in the base metal to form the metal carbide layer. In another example, a bath of fused boric acid or borate' such as sodium borate, i.e., borax, containing the metal element is prepared and the mold'member is then inserted into the fused bath for a period of time to obtain the diffusion of the metal into the surface layer. If desired, an electrolytic treatment of the mold member i'nthe fused bath can be utilized by making the mold member a cathode and such a treatment enables the obtaining of the metal carbide layer in a shorter period of time. v, I

The thickness of the carbide layer formed on the mold surface maybe between 2 to 3 microns toabout 100 microns; The carbide, layer with a thickness of less than 2 microns will have insufficient corrosionresistance and wear resistance and the life of the mold may be shortened. A carbide layer with a thickness greater than 100 microns will have less resistance to thermal shock and will easily peeloff of the mold.

. least two mold members of ferrous alloy having'surfaces coacting to form a mold cavity, pouring the molten aluminum or aluminum alloy into the mold cavity, and allowing the molten metal to solidify with the im- When removed from the bath, the core pin was cooled by oil quenching and any fused bath material adhering to the core pin was removed by washing in warm water.

Then the treated pin was subjected to a tempering in charcoal powder at 550C. for a period of 1 hour.

A part of this pin was cut off and the base metal and the metal carbide layer were examined by a microscopic observation, X-ray micro'analysis and X-ray diffration. The results of the microscopic observation of a cross-section is shown in photomicrograph (400 X) which is illustrated in FIG. 2. As shown in the photomicrograph, a surface layer 51 which has a thickness of approximately 7 microns and which layer is a layer of niobium carbide (NbC) containing a small amount of iron was disposed on the base metal or substrate 52. A

small amount of tantalum was also contained in the carbide layer 51. The carbide layer had a Vickers hardness of 2600 and the base metal had a Rockwell hardness" (Scale C) of 45. Thus, the carbide layer was found to be extremely hard,

When casting aluminum in die casting mold with a conventional pin mold of an annealed and tempered alloy tool steel (SKD 61), approximately 1,000 shots could be made before adhesion of aluminum occurred at the pin head and casting had to be stopped A mold using the movable pin produced according to the pres;

ent invention'and 'by this example could be continually used up to about 5,000 shots without adhesion of aluminum. Moreover, since there was no adhesion of aluminum on the pin produced according tothe present invention, the dimensional accuracy of the hole made by this pin was extremely excellent and therewas almost no wear in the sliding parts of the pin;

EXAMPLE 2 A core pin for gravity casting having a diameter of 23 mm and a length of 180 mm and a spool core for die casting having a diameter of 80 mm and a length of 80 mm were made from an annealed alloy tool steel (81(1) 61) and a metal carbide layer was formed thereon by l the same method as inExample 1. The core pin and provement comprising using a casting mold having portions of the surface of the mold members forming the least one metal selected from Group V, Subgroup aof the periodic table. t I I The present invention is described in more detail by the following examples:

EXAMPLE 1 movable core pin.

mold cavity provided with a metal carbide layer of at spool core were assembled into their separate mold and an aluminum alloy was cast.

The pin for the gravity casting was still usable even after 3,000 shots. in the spool core, aluminum adhesion appeared only after 2,000 shots and the adhered aluminum was easily removed by brushing the core in the mold to make the core usable. In contrast, a'conventional core pin for gravitycasting and aspool core for die casting made from annealed alloy tool steel (SKD 61) could not be used after the 1,000 shot, respectively, due to the adhesion of aluminum,

EXAMPLE 3 A core pin having dimensions of approximately 80 X 120 X 170 mm and having a cavity was made from an I annealed alloy tool steel (SKDGI). It was treated by The .core pin was made from an annealed alloy tool steel (Japanese lndustrial Standard SKD 61). The core pi'n was provided with the layer of metal carbide by being immersed in a bath of fused borax (Na B O which was mixed with 20 percent ferroniobium powder (containing 63 percent niobium and 6 percent tantalum) for a period of 8 hours at a temperature of 950C. 4

being packed in a steel case containing a powdering treatment material consisting of powder .of percent ferro vanadium.(containing approximately 52 percent vanadium) and 20 percent potassium tetrafluoroborate (K31 The case with its contents was heated to 1,000C. in an electric furnace for a period of 6 hoursl it should be noted that at this temperature, the potassium tetrafluoroborate will activate the metal. After the v 6 hour period of'heating', the steel case with the core area pin was taken out and cooled in air to obtain a core pin for use in a mold according to the present invention.

A part of the core pin was cut off and the surface layer was examined just as in Example 1 by a microscope, X-ray microanalyzer and X-ray diffraction. As a result of the examination, the layer had a thickness of approximately 12 microns and was a vanadium carbide (VC) layer containing almost no iron.

A core pin formed by the above treatment was assembled in the mold, and the casting operation was performed. As a result of the casting, there was no aluminum adhesion on the core pin of the present invention up to 5,000 shots. In contrast, a conventional core pin made from conventional quenched and tempered alloy tool steel (SKD 61) had adhesion of aluminum on the projected portions of the mold surface after the 2,000th shot. Thus, the test confirmed the durability of the mold having a vanadium carbide layer as being superior to the conventional core pin just'as the niobium carbide layer proved to be superior to the conventionalv mold.

EXAMPLE 4 A fixed die and a movable die suchas dies 1' and 2 of the mold shown in FIG. 1 were made from an annealed alloy tool steel (SKD 6l Using the same method as in Example 3, thedies 1 and 2 were packed cellent durability and a casting method requiring very the adhesion of aluminum, and also has excellent wear resistance due to the hardness of the metal carbide layer, the present invention produces a mold with exlittle repair work and which casting method produces castings with smooth surfaces.

What we claim is:

1. In a mold for casting aluminum or aluminum alloys comprising at least two mold members having mold surfaces coacting to form a cavity for receiving the molten aluminum or aluminum alloy, each of said mold members being of a ferrous metal, the improvement comprising at least a: portion of the mold surface of at least one mold member being a metal carbide layer of in two iron cases with each case containing a powdery treatment material consisting of 80 percent ferrovanadium and 20 percent potassium tetrafluoroborate. Each case was heated for 6 hours at 1.,000C. in an electric furnace. At the end ofthe 6 hour period, the cases were taken out of the furnace and cooled in air. The surfaces of dies 1 and 2 each had a vanadium carbide (VC) layer. The carbide layer, which was both dense and uniform, was approximately 11 microns in thickness.

The present invention relates to a mold for use in a method of casting aluminum or aluminum alloy which mold is made of ferrous basemetal and has a portion or the entire mold surface provided with a metal carbide layer with the metal selected from Group V, Subgroup a of the periodic table. Since the metal carbide layer formed on the mold surface has excellent corrosion resistance to molten aluminum, serves to reduce at least one metal being selected from Group V, Subgroup a of the periodic table.

2. In a mold according to claim I, wherein the metal carbide is vanadium carbide.

3. In a mold according to claim 1, wherein the metal carbide is a niobium carbide.

4. In a mold according to claim I, wherein the metal carbide is tantalum carbide.

.5. In a mold according to claim 1, wherein saidmetal carbide layer is a diffusion layer.

6. In a mold according to claim 1, wherein said mold members include at least one core. pin having a surface layer of said metal carbide.

7. In a mold according to claim 1, wherein the entire surface of the mold members are provided with said metal carbide layer.

. 8. In a mold according to claim 1, wherein said metal carbide layer has a thickness in the range of 2 to microns. r

, 9. In a method of casting molten aluminum or aluminum alloys comprising providing a casting mold formed of atleast two mold members of a ferrous alloy having surfaces coacting to form a mold cavity, pouring the molten aluminum or aluminum alloys into the mold cavity, and allowing the molten metal to solidify, the improvement comprising using a casting mold having a portion of the surface. of the mold members forming the mold cavity provided with a metal carbide layer of at least one metal selected from Group V, Subgroup a of the periodic table. 

1. IN A MOLD FOR CASTING ALUMINUM OR ALUMINUM ALLOYS CMPRISING AT LEAST TWO MOLD MEMBERS HAVING MOLD SURFACES COACTING TO FORM A CAVITY FOR RECEIVING THE MOLTEN ALUMINUM OR ALUMINUM ALLOY, EACH OF SAID MOLD MEMBERS BEING OF A FERROUS METAL, THE IMPROVEMENT COMPRISING AT LEAST A PORTION OF THE MOLD SURFACE OF AT LEAST ONE MOLD MEMBER BEING A METAL CARBIDE LAYER OF AT LEAST ONE METAL BEING SELECTED FROM GROUP V, SUBGROUP A OF THE PERIODIC TABLE
 2. In a mold according to claim 1, wherein the metal carbide is vanadium carbide.
 3. In a mold according to claim 1, wherein the metal carbide is a niobium carbide.
 4. In a mold according to claim 1, wherein the metal carbide is tantalum carbide.
 5. In a mold according to claim 1, wherein said metal carbide layer is a diffusion layer.
 6. In a mold according to claim 1, wherein said mold members include at least one core pin having a surface layer of said metal carbide.
 7. In a mold according to claim 1, wherein the entire surface of the mold members are provided with said metal carbide layer.
 8. In a mold according to claim 1, wherein said metal carbide layer has a thickness in the range of 2 to 100 microns.
 9. In a method of casting molten aluminum or aluminum alloys comprising providing a casting mold formed of at least two mold members of a ferrous alloy having surfaces coacting to form a mold cavity, pouring the molten aluminum or aluminum alloys into the mold cavity, and allowing the molten metal to solidify, the improvement comprising using a casting mold having a portion of the surface of the mold members forming the mold cavity provided with a metal carbide layer of at least one metal selected from Group V, Subgroup a of the periodic table. 